Glass-like coatings with oxidation-promoting properties

ABSTRACT

Metallic substrates such as steel plate are provided with a glass-like silicate coating which promotes oxidation by applying thereto a slip comprising a frit, about 5 to 30 percent of alumina and up to about 8 percent of a clay based on the weight of the frit. The frit contains about 0.3 to 6 percent by weight, calculated as the oxide, of a heavy metal such as cobalt, nickel, manganese, copper and/or iron, and combined sulfur present in about 0.3 to 7.5 percent by weight calculated as barium sulfide. The coated plate is then fired. A preferred frit composition comprises, by weight: SiO2 - 40 to 60 percent, B2O3 - 0 to 10 percent, Al2O3 - 0.5 to 7.5 percent, ALKALI METAL OXIDES - 10 TO 25 PERCENT, ALKALINE EARTH METAL OXIDES - 5 TO 27 PERCENT, HEAVY METAL OXIDES - 0.3 TO 6 PERCENT, AND FLUORINE FLUORINE - 0 TO 5 PERCENT, AND SULFUR CALCULATED AS BaS - 0.3 to 3 percent.

GLASS-LIKE COATINGS WITH OXIDATION-PROMOTING PROPERTIES Inventors: HansKyri; Albert Reiss, both of Koeln; Josef Suss, Leverkusen-Rheindorf, allof Germany [73] Assignee: Bayer Rickmann GmbH, Koeln,

Germany [22] Filed: July 7, 1972 [21] Appl. No.: 269,700

[30] Foreign Application Priority Data July 17, 1971 Germany P 21 35835.0

[52] US. Cl 117/129, 106/48, 126/19 [51] Int. Cl A2lb l/ [58] Field ofSearch 117/129; 106/48, 106/52; 126/19; 252/428, 439

[56] References Cited UNITED STATES PATENTS 3,580,733 /1971 Ott ll7/l293,671,278 6/1972 Borowski ll7/l29 X 3,565,830 2/1971 Keith et al.252/439 X [4 1 Oct. 16,1973

Primary E.\'aminerAlfred L. Leavitt Assistant Examiner-David A. SimmonsAttorney-Ralph D. Dinklage et al.

[5 7] ABSTRACT Metallic substrates such as steel plate are provided witha glass-like silicate coating which promotes oxidation by applyingthereto a slip comprising a frit, about 5 to percent of alumina and upto about 8 percent of a clay based on the weight of the frit. The fritcontains about 0.3 to 6 percent by weight, calculated as the oxide, of aheavy metal such as cobalt, nickel, manganese, copper and/or iron, andcombined sulfur present in about 0.3 to 7.5 percent by weight calculatedas barium sulfide. The coated plate is then fired. A preferred fritcomposition comprises, by weight:

SiO to percent, B 0 0 to 10 percent, A1 0 0.5 to 7.5 percent, alkalimetal oxides 10 to 25 percent, alkaline earth metal oxides 5 to 27percent, heavy metal oxides 0.3 to 6 percent, and fluorine fluorine 0 to5 percent, and sulfur calculated as BaS 0.3 to 3 percent.

7 Claims, No Drawings GLASS-LIKE COATINGS WITII OXIDATION-PROMOTINGPROPERTIES This invention relates to glass-like coatings withoxidation-promoting properites for aritcles of metal, in particulariron.

In baking, roasting or grilling, deposits of fat, protein andcarbohydrates accumulate in the ovens of cookers, and these areextremely difficult to remove by conventional domestic cleaning methods.In general, the ovens have an enamel coating which is seriously damagedby this cleaning, which is predominantly mechanical. Deposits which areformed subsequently adhere all the more firmly to the roughened, damagedenamel surface and can no longer be completely removed. Accordingly, theovens assume an unhygienic appearance.

Deposits of a different kind, but with similarly unpleasant properties,are formed in the exhaust systems of internal combustion engines. Sincethese resin-like carbonized deposits cannot, as a rule, be completelyremoved and since, as already mentioned, even their partial removal mayresult in damage to the substrate, many attempts have been made toreplace chemical and mechanical methods of cleaning. For example, it ispossible to obtain cookers whose ovens can be heated after use totemperatures of 500C. and higher. As a result, the deposits arecarbonized and can thus be removed. However, the outlay which thisinvolves is considerable, especially since the cookers must beeffectively isolated from their surroundings. It has also been proposedthat the gases given off during heating may be passed through a secondcombustion chamber which is provided with a catalytically activesurface. In this way it is intended that the exhaust gases should becompletely burnt to avoid pollution.

In another process, the formation of deposits is avoided while the ovenis actually in use. The surface of the ovens are provided withoxidation-promoting agents which enable organic materials to be burnt attemperatures as low as 200 to 300C. The oxidation catalysts can eitherbe applied to the surface of a supporting material or embedded in anenamel coating. The supporting materials are generally oxidic materialsof ceramicor glass-like composition. The catalytically active substancesused are either metals, preferably noble metals, or metal oxides. Thus,for example, enamel coatings which contain from to 70 percent by weightof metal oxides are known. A content of percent by weight of thecatalytically active oxide, based on the total weight of the finishedenamel layer, is generally encountered in practice. The oxides ofcobalt, manganese, copper and chromium have been noted as examples ofcatalytically active substances. One considerable economic disadvantageof these oxidation-promoting coatings is the high content of metaloxide.

One feature common to all oxidation-promoting coatings is the productionof an effective surface which is extremely rough, i.e., the specificsurface is greatly increased compared with a smooth surface.Accordingly, it is essentially an application of experience gained inthe art of catalytic processes.

Ground costs for steel plate contain oxides of cobalt, nickel, manganeseor copper as adhesion promoters. As a rule, the total quantity f0 theseoxides only occasionably exceeds 4 percent. It is known that theseadhesive oxides lose some or all of their effect if sulfides are presentin the enamel, depending upon the sulfuric content. It is assumed thatif sulfides of the metals are in the adhesive layer these will no longerhave the property of promoting adhesion.

The present invention relates to glass-like mixtures for the productionof oxidation-promoting coatings on metallic substrates, in particular onsteel plate, which coatings contain, in a predominantly silicate matrix,compounds of the heavy metals cobalt, nickel, manganese, copper and/oriron in a quantity of from about 0.3 to 6 percent by weight, calculatedin terms of the oxides, and sulfuric in the form of sulfide in aquantity of from about 0.3 to about 7.5 percent by weight, calculated interms of barium sulfide.

It has surprisingly been found that the catalytic effect of heavy metaloxides in glass-like coatings in respect to the combustion of organicvapors and deposits is not diminished, but on the contrary isconsiderably intensified in the presence of sulfides. Compared withconventional processes, therefore, the content of the heavy metalcompounds can be considerably reduced. In general, a quantity of up toabout 5.0 percent by weight, calculated in terms of the oxides, issufficient. Larger quantities are not economical, although they are notharmful. The optimum catalytic effect of the heavy metals is developedwhere the atomic ratio in the frit of the divalent heavy metals to thesulfidic sulfur is from about 1:08 to 1:12, with an oxide content in thefinished coating (enamel) of from about 3 to 6 percent by weight.

Frits of about the following composition are suitable for producingthese glass-like coating with the self cleaning properties:

SiO 40 to 60 percent by weight;

B 0 0 to 10 perent by weight;

A1 0 0.5 to 7.5 percent by weight;

Alkali metal oxides (Li O Na O K 0 10 to 25 percent by weight;

Alkaline earth metal oxides (Ca0 SrO BaO) 5 to 27 percent by weight;

F 0 to 5 percent by weight;

S 0.3 to 3 percent by weight;

Heavy metal oxides (FeO C00 NiO MnO CuO)0.3 to 6 percent by weight.

It is possible by choosing the starting materials in the known mannerwithin these composition ranges to produce glass-like coatings whichhave desired firing temperatures and adhesion properties for metallicsubstrates (enamels). One important feature in preparing the frits forthis purpose is the addition of sulfides or of sulfur compounds whichform sulfides during the fusion process. It is advantageous to providereducing conditions during fusion of the frits. In order to avoidundesirable losses of sulfur by oxidation, the mixture of startingmaterials should contain as few oxidizing constituents as possible. Ithas proved to be particularly advantageous to add reducing materials,for example carbon powder, to the mixture of starting materials in orderto keep all the sulfur in sulfidic form or, where sulfur itself isadded, to convert it into sulfidic sulfur. Apart from sulfur, it ispossible to use any starting materials which are able to form sulfideswith the other constituents during the fusion process and which, inaddition, do not interfere in any way with formation of the glass-likecoating compositions having the aforementioned composition. In additionto elemental sulfur, the alkaline earth metal sulfides can be readilyused. Alkali metal sulfides or heavy metal sulfides can also be used.

In other respects, the frits are prepared with starting materialscommonly used in the enamelling industry at fusion temperatures of fromabout 1100 to 1400 C. For application, the ground or flaked frits arefurther ground in a mill in which clay, set-up salts and water are addedto the slip. Hydrated alumina or aluminum oxide are also added in themill in a quantity of from about 5 to 30 percent by weight, based on thetotal weight of the frit. Some of the heavy metal oxides can also beadvantageously added during the preparation of the slip. Thecatalytically active surface is enlarged by the addition of the hydratedalumina or aluminum oxide. It has been noted that on firing, the oxidicconstituents of the mill clay form a substantially homogeneousglass-like mass with the constituents of the frit, in the same way asthe A1 The slip is applied to the substrate to be coated by the usualmethods of spraying, dipping or casting. The tiring temperature isgoverned both by the composition of the frit and by the type ofsubstrate. In general, it is in the range from about 780C. Articles ofsteel plate or iron are preferably coated with a ground coat beforehand.The ground coats containing adhesive oxides known in the enamel industryare suitable for this purpose.

The invention is illustrated by the following Examples:

EXAMPLE 1 From a frit produced from a starting mixture having thefollowing analysis:

parts by weight SiO, 47.63 A1 0 2.88 Na,0 13.8 K 0 0.34 CaO 26.80 82187.0 C00 9 B210 0.8 Total: 100. IS

(the C00 in the above frit can be replaced by approximately 1.2 parts byweight of C08), a slip is prepared, the mill batch having the followingcompositions:

parts Frit I 100 Clay 4.0 Hydrated alumina 30.0 C00 4.0 Water 65 .0

This is applied by casting onto a suitably pretreated, ground-coatedsteel plate. After drying and firing at the normal enamel-firingtemperatures of around 800C, a coating of the following chemicalcomposition is obtained:

Coating Example 1 parts by weight SiO, 39.1 A1,0, 18.8 N310 10.9 x,o 0.3CaO 21.2 BaO 5.5 C00 3.9 S'- 0:6 Total: 100.3

EXAMPLE 2 A slip is prepared as described in Example 1 the mill batchhaving the same composition but the frit having been prepared from astarting mixture having the following composition:

parts by weaighlt SiO, o 1 8.8 it? 533 2 CaO 26.80 BaS 7 .0 NiO 0.9 8600.8 Total: 104.64

This is applied to prepared substrates. Favorable results were alsoobtained where the NiO was replaced by 1.2 parts by weight of NiS.

The composition of the coating obtained after firing at the sametemperatures in Example 1 is as follows:

parts by weight SiO 39.] A1 0, 18.8 Na O 10.9 K 0 0.3 CaO 21.2 BaO 5.5C00 3.16 NiO 0.72

0.6 Total: 100.28

EXAMPLE 3 A slip is prepared as in Examples 1 and 2 from a frit preparedfrom a starting mixture having the following composition:

parts by weight SiO, A1 0, 2.8 N810 13.3 K 0 0.33 CaO 25.8 8218 6.8 MnO1.3 BaO 0.8 Total: 100.33

This is applied to a steel plate substrate. Favorable results were alsoobtained where the MnO was replaced by 1.3 parts by weight of MnS. Afterfiring a coating of the following composition is obtained:

EXAMPLE 4 A frit prepared from a mixture of starting materials havingthe following oxidic analysis was used as described in Example 1:

parts by weight SiO, 50 A1 0, 2.7 Na O 13.0 K 0 0.3 CaO 25.3 BaS 6.6C00,, 7 0.9 BaO 0.75 Total: 99.95

EXAMPLE 5 A frit prepared from a mixture of starting materials havingthe following analysis (expressed as oxides) was used as described in-Example 1:

parts by weigh SiO 58.5 Al,O Na.,0 K,0 BaO FeO CaO Total A mill batch asdescribed in Example 1 is prepared therefrom and an enamel coating ofthe following composition subsequently obtained:

The following mixture of starting materials are processed into a frit asdescribed in Example 1:

parts by weigh SiO, 49.5 Al,0 K,O Na,0 BaO CaO B 0, ZrO, FeO

Total:

Using a mill batch as described in Example 1 but with a slightlyincreased cobalt oxide content (4.9 parts per 100 parts of frit), acoating of the following composition is obtained:

parts by weight SiO, 39. A10, Na,0 K,O BaO CaO 3,0, ZrO, FeO C Total:

The coatings obtained after firing on ground-coated plates as describedin Examples 1 to 6 have the same roughness as a fineto average-grainabrasive paper and show gradations in color from dark grey throughblack-grey to black.

The considerable porosity of these coatings is reflected in the factthat drops of a liquid applied to them are quickly absorbed.

The cleaning effect of the enamel layers prepared as described in thepreceding Examples was tested and assessed as follows:

10 X 10 cm plates were enamelled as described above. After heating to200 C. these plates were coated with standard edible oils and moltenedible fats by means of a striped brush. Thereafter they were heated tobetween 250 and 300C for 1 to 2 hours.

After heating, the plates were cooled and assessed on the followingscale.

The plate or layer of enamel does not show any discoloration, spots orresidues over the coated areas:

assessment: very good The enamel shows hardly noticeable discolorationand edge marks in the areas coated:

assessment: good The enamel shows slight discoloration and noticeableedges:

assessment: satisfactory The enamel is darker in the coated areas andshows dull residues which are easy to remove mechanically:

assessment: adequate The enamel is heavily discolored in the areascoated and shows residues in the form of bright speckles which areeither extremely difficult or totally impossible to remove by mechanicalmeans:

assessment: inadequate in order to be recognized as serviceable a coatedenamel should at least be assessed as satisfactory after 10 tests.

The enamels prepared as described in Example 1 to 6 were subjected to atleast 12 tests and showed results ranging from very good tosatisfactory.

It will be appreciated that the instant specification and examples areset forth by way of illustration and not limitation, and that variousmodifications and changes may be made without departing from the spiritand scope of the present invention.

What is claimed is:

l. A metallic substrate coated with a glass-like silicate coatingproduced by applying to the substrate a composition comprising apredominantly silicate matrix, a compound of at least one heavy metalselected from the group consisting of cobalt, nickel, manganese, copper,and iron, said heavy metal compound being present in about 0.3 to 6percent by weight calculated as the oxide, and sulfer present as sulfidein about 0.3 to 7.5 percent by weight calculated as barium sulfide andfiring the substrate.

2. A coated substrate according to claim 1 wherein to the frit prior toapplication to the substrate there is added about 5 to 30 percent ofaluminum oxide based on the weight of the frit.

3. A coated substrate according to claim 2 wherein the frit has theapproximate composition by weight calculated as the oxides of Si0 40 to60 percent.

B 0 0 to 10 percent,

A1 0 0.5 to 7.5 percent,

alkali metal oxides 10 to 25 percent,

alkaline earth metal oxides 5 to 27 percent,

heavy metal oxides 0.3 to 6 percent, and

fluorine 0 to 5%, and

sulfur calculated as BaS 0.3 to 3%.

4. The process for the production of a metallic substrate coated with aglass-like silicate coating comprising forming a slip comprising apredominantly silicate matrix, a compound of at least one heavy metalselected from the group consisting of cobalt, nickel, manganese, copper,and iron, said heavy metal compound being present in about 0.3 to 6% byweight calculated as the oxide, and sulfur present as sulfide in about0.3 to 7.5 percent by weight calculated as barium sulfide, based on thebalance of the solids in said slip calculated as oxides, said slipfurther containing about to 30 percent by weight of aluminum oxide andup to about 8 percent by weight of clay, forming a coating of said slipon said substrate, and firing the coated substrate.

5. The process of claim 4 wherein said slip is prepared from saidaluminum oxide, said clay and a frit having an approximate compositionby weight calculated as the oxides of SiO 40 to 60 percent,

B 0 O to 10 percent,

A1 0 0.5 to-7.5 percent,

alkali metal oxides 10 to percent,

alkaline earth metal oxides 5 to 27 percent,

heavy metal oxides 0.3 to 6 percent, and

fluorine 0 to 5 percent, and

sulfur calculated as BaS 0.3 to 3 percent.

6. The process of claim 5 in which at least a portion of the heavy metaloxide is added during the preparation of the slip.

7. The process of claim 5 in which the metallic substrate is coated witha ground coat prior to application of said slip to the metallicsubstrate.

2. A coated substrate according to claim 1 wherein to the frit prior toapplication to the substrate there is added about 5 to 30 percent ofaluminum oxide based on the weight of the frit.
 3. A coated substrateaccording to claim 2 wherein the frit has the approximate composition byweight calculated as the oxides of SiO2 - 40 to 60 percent. B2O3 - 0 to10 percent, Al2O3 - 0.5 to 7.5 percent, alkali metal oxides - 10 to 25percent, alkaline earth metal oxides - 5 to 27 percent, heavy metaloxides - 0.3 to 6 percent, and fluorine - 0 to 5%, and sulfur calculatedas BaS - 0.3 to 3%.
 4. The process for the production of a metallicsubstrate coated with a glass-like silicate coating comprising forming aslip comprising a predominantly silicate matrix, a compound of at leastone heavy metal selected from the group consisting of cobalt, nickel,manganese, copper, and iron, said heavy metal compound being present inabout 0.3 to 6% by weight calculated as the oxide, and sulfur present assulfide in about 0.3 to 7.5 percent by weight calculated as bariumsulfide, based on the balance of the solids in said slip calculated asoxides, said slip further containing about 5 TO 30 percent by weight ofaluminum oxide and up to about 8 percent by weight of clay, forming acoating of said slip on said substrate, and firing the coated substrate.5. The process of claim 4 wherein said slip is prepared from saidaluminum oxide, said clay and a frit having an approximate compositionby weight calculated as the oxides of SiO2 - 40 to 60 percent, B2O3 - 0to 10 percent, Al2O3 - 0.5 to 7.5 percent, alkali metal oxides - 10 to25 percent, alkaline earth metal oxides - 5 to 27 percent, heavy metaloxides - 0.3 to 6 percent, and fluorine - 0 to 5 percent, and sulfurcalculated as BaS - 0.3 to 3 percent.
 6. The process of claim 5 in whichat least a portion of the heavy metal oxide is added during thepreparation of the slip.
 7. The process of claim 5 in which the metallicsubstrate is coated with a ground coat prior to application of said slipto the metallic substrate.